1. Field of the Invention
The present invention relates to high thermal conductivity pastes which are used as a heat transfer means for cooling electronic components such as VLSI chips and, in particular, to a specific dispersant used in the paste to provide a paste having enhanced thermal conductivity and paste properties.
2. Description of Related Art
Electronic devices and components generate heat and in many applications the heat needs to be effectively dissipated for the device to function. In integrated circuit chips mounted in an array on substrates, the need to achieve greater computer speeds and the miniaturization of electronic devices has resulted in components which generate more heat and which require denser packing of the active components of the electronic device. The use of such components and denser packing is only practicable, however, when the heat generated is effectively removed from the active components and a number of techniques have been developed to cool such electronic components.
Liquid coolants are commonly employed in such high density cooling applications and various systems have been disclosed in the art for providing a separate cooling member to cool the component. Fins, channels, baffles, and other heat exchanger type devices have been developed to remove heat from the electronic component. Exemplary apparatus for cooling integrated circuit chips are shown in U.S. Pat. Nos. 5,177,667 and 5,294,830, the disclosures of both patents being herein incorporated by reference.
In most of these cooling devices there is an air space between the electronic component to be cooled and the cooling device which air space has a relatively low heat transfer rate. To overcome this problem, thermal compounds commonly referred to as thermal greases or thermal pastes have been developed to enhance the transfer of heat from the electronic component to the cooling device by contacting and connecting both surfaces and providing a high heat transfer conducting means. The thermal paste must have high thermal conductivity and preferably be an electrical insulator. The paste must also have a low viscosity and be compliant so that the thermal paste can be easily applied to the surfaces of the electronic component to be cooled and can conform with the microscopically rough surface of the electronic component which are often bowed or tilted to minimize air gaps which are detrimental to the cooling process. Low viscosity is also important because the electronic components and the solid bonds that attach the components to a substrate are fragile and the force needed to apply the thermal paste must be minimal. It is a further requirement that the thermal paste be able to withstand power cycling and the mechanical stresses arising from the differences in coefficient of thermal expansion of the electronic component, paste and cooling system over the life of the component without the paste degrading significantly in thermal conductivity or mechanically, such as experiencing phase separation between the liquid and solid components of the paste.
A number of thermal pastes have been developed to be used in electronic systems and, in general, the thermal paste comprises thermally conductive filler particles in a liquid dielectric medium along with other components such as dispersants, antioxidants, and stabilizers.
U.S. Pat. Nos. 5,094,769 and 5,213,704 provide a phase-stable, thermally conductive paste having a relatively constant thermal conductivity and viscosity for transferring heat from electronic components. A critical component of the paste is the use of a coupling agent such as an organosilane which has a functionality which is reactive with the thermally conductive filler particles and a functionality which is of similar polarity to the liquid carrier. The disclosures of these patents are hereby incorporated by reference.
In U.S. Pat. No. 5,098,609 stable high solids, high thermal conductivity pastes are disclosed which contain a stabilizing dispersant to inhibit liquid-solids separation and which facilitate incorporation of more solid fillers into the paste. Examples of stabilizing dispersants are monomeric acids; monomeric, oligomeric and/or polymeric fatty acid esters and salts thereof; petroleum sulfonates; and polyimides. Exemplary of fatty acid esters are the reaction products of hydroxystearic acid (HSA) and a 5-18 carbon acid or acid chloride. Preferred dispersants are fatty acid esters which are the reaction product of 12 hydroxy stearic acid (12 HSA) and a 5-10 carbon aliphatic acid chloride or 5-18 carbon alicyclic acid, chloride or aromatic acid chloride. 12 HSA reacted with itself to yield a polymeric/oligomeric material is also disclosed to be useful. Polyesters as disclosed in U.S. Pat. No. 3,778,287 to Stansfield and Hypermers made by ICI are disclosed in the patent as being useful as the stabilizing dispersant. The disclosures of both patents are hereby incorporated by reference.
Increasing demands of industry however, are requiring, pastes having improved thermal conductivity in order to be used with the new computers and other electronic components now being developed. In general, higher filler loadings are needed for higher thermal conductivity pastes and while pastes having thermal conductivities ranging from 2.2-4.9 W/mK( watts/meter-degree K ) are available the pastes tend to be too thick and stiff at the high filler loadings required for thermal conductivities greater than 3W/mK. Hard, non-compliant and difficult to dispense pastes can not be used for most applications. In addition, commercial pastes and dispersants have shown higher and variable levels of ionics, particularly C1-, which presents a corrosion concern.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a thermally conductive paste which may have a wide range of filler loading levels necessary to provide a wide range of thermal conductivity properties With relatively low paste viscosity, compliance, low ionics, paste phase stability and high electrical resistivity.
It is another object of the present invention to provide a method for cooling an electronic component comprising the use of a specially formulated thermally conductive paste to provide a thermal transfer means between the electronic component and the cooling mechanism.
It is yet another object of the present invention to provide high thermal conductivity compliant pastes using 12-hydroxy stearic acid derived polymeric dispersants with Acid Nos. in the range of 30-100.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.